You drink moderately and still suffer. Here's the biological reason.

ADH1B encodes alcohol dehydrogenase, the enzyme that converts ethanol (the alcohol in your drink) into acetaldehyde, a toxic intermediate. This is the first critical step in alcohol metabolism. In most people, this enzyme works efficiently and the acetaldehyde is quickly passed to the next enzyme for further processing.

If you carry the ADH1B His48Arg variant (Arg/Arg), you’re a fast converter. Roughly 20% of people with European ancestry carry this variant, while about 70% of people with East Asian ancestry do. Fast converters produce acetaldehyde rapidly, which means acetaldehyde builds up in your blood quickly, causing facial flushing, nausea, and hangover symptoms even from small amounts of alcohol.

You feel the effects almost immediately after drinking. Your face flushes. You feel nauseated or dizzy. The next morning, you’re exhausted and foggy because your body spent the night processing a toxic buildup of acetaldehyde. The problem isn’t that you’re drinking too much; it’s that you’re creating acetaldehyde faster than the next enzyme in the chain can clear it.

ALDH2 encodes acetaldehyde dehydrogenase, the enzyme that converts toxic acetaldehyde into acetate, which your body can use as fuel or excrete. This is the critical second step, and it’s the biggest bottleneck in alcohol metabolism. If acetaldehyde can’t be cleared, it accumulates in your blood and tissues, triggering all the worst hangover symptoms.

The ALDH2 *2 allele (Glu487Lys) dramatically reduces or nearly eliminates the enzyme’s function. This variant is present in roughly 35 to 40% of people with East Asian ancestry and is very rare in people with European ancestry. If you carry the *2 allele, your ALDH2 enzyme is working at a fraction of normal capacity, meaning acetaldehyde builds up rapidly and persists in your blood even from small amounts of alcohol.

You don’t just feel hungover the next day; you feel poisoned. Severe headache, nausea, weakness, and brain fog can last 24 to 48 hours after one or two drinks. You might also experience flushing, chest pain, or heart palpitations. Your body is literally drowning in acetaldehyde because it can’t be cleared efficiently.

CYP2E1 is a microsomal enzyme that metabolizes ethanol via a secondary pathway. This pathway generates reactive oxygen species (free radicals) as a byproduct. Most of the time, your antioxidant systems handle this fine. But certain variants in CYP2E1 increase the rate of free radical production, which means alcohol consumption generates excessive oxidative stress in your liver cells.

CYP2E1 variants affect how much oxidative damage is generated from each drink. If you carry variants that increase CYP2E1 activity, drinking alcohol creates significantly more liver cell damage and free radical stress than it does in people without those variants. The damage accumulates with each drink, even if you never reach dangerous levels of alcohol consumption.

You feel the effects as systemic fatigue, brain fog, and a general sense of being unwell. Your body is working overtime to repair oxidative damage. The hangover doesn’t just feel like dehydration; it feels like your entire system was stressed and inflamed by a metabolic process you couldn’t control.

GSTM1 encodes glutathione S-transferase mu 1, an enzyme that attaches glutathione (your body’s master antioxidant and detox molecule) to acetaldehyde and other toxic alcohol metabolites, making them easier for your liver and kidneys to excrete. This is a critical cleanup step. Without it, toxic compounds linger in your system longer.

Roughly 50% of people carry a GSTM1 null genotype, meaning they have a complete deletion of the GSTM1 gene and produce no functional GSTM1 enzyme. If you’re GSTM1 null, your body has significantly reduced capacity to detoxify acetaldehyde and other alcohol-derived toxins, prolonging hangover symptoms and increasing inflammation.

You experience hangovers that last much longer than they should. The nausea, fatigue, and brain fog don’t resolve after 12 to 24 hours; they linger for days. This isn’t because you drank a lot; it’s because your body can’t clear the toxic byproducts efficiently.

SOD2 encodes superoxide dismutase 2, a mitochondrial antioxidant enzyme that neutralizes superoxide radicals before they cause damage to your cell membranes and DNA. This is one of your body’s most important defenses against oxidative stress. Alcohol metabolism generates substantial free radical production, especially in the mitochondria of your liver cells. If SOD2 isn’t working optimally, that oxidative damage accumulates rapidly.

SOD2 variants (such as Ala16Val, rs4880) affect the efficiency of this antioxidant. People with the Val/Val genotype have reduced mitochondrial antioxidant capacity, meaning alcohol-induced free radical damage accumulates faster and causes more cellular stress. The effect is most pronounced in your liver and brain.

Your hangover symptoms include not just headache and nausea but also severe brain fog, difficulty concentrating, and a sense of systemic depletion. This is because the free radical damage from alcohol metabolism is overwhelming your mitochondrial defenses, leaving your cells literally burned out.

TNF encodes tumor necrosis factor alpha, a pro-inflammatory cytokine that coordinates your immune response to injury and stress. Alcohol metabolism generates cellular damage and triggers the release of acetaldehyde-derived damage-associated molecular patterns (DAMPs) that activate TNF signaling. In people with TNF variants, this inflammatory response is amplified, turning a manageable metabolic stress into a full-blown inflammatory cascade.

TNF variants (such as -308G>A, rs1800629) affect baseline TNF production levels. People carrying the A allele have higher TNF baseline expression, meaning alcohol triggers a disproportionately strong inflammatory response even from small amounts of alcohol. The inflammation drives much of the hangover symptom severity.

You don’t just feel tired the next day; you feel inflamed. Achy joints, overall body pain, brain fog, and a sense of being unwell dominate your hangover. The inflammation can persist for 24 to 48 hours, even though the acetaldehyde was cleared from your system hours earlier.